The present invention relates to communication systems and is particularly directed to a system for counteracting the non-linear behavior of power amplifiers, such as may be used in microwave communication.
Transmission of radio signals over the (i.e., use of a wireless link) air provides many advantages over other transmission media for communication systems. Radio systems do not require installation of a transmission medium between stations, avoiding the expense of installing and maintaining fiber optic or electrical cables. Thus, communication systems using radio links are particularly well suited to installations where adverse/hostile terrain, existing infrastructures and/or regulations make installation of cable, or other media, prohibitive.
A problem inherent in many radio systems, particularly those operating in the 20-40 GHz range, is variable path fade caused by changing atmospheric and environmental conditions, such as precipitation and varying foliage at microwave facility sites and along transmission paths. It should be appreciated that the propagation loss for high frequencies, such as the aforementioned 20 to 40 GHz systems, varies greatly depending on such dynamic circumstances. For example, from a clear to rainy day signal attenuation or signal loss variation can be as much as 40 dB per km. Accordingly, in order to ensure a sufficient signal margin to consistently provide an adequate signal-to-noise ratio during periods of extreme signal attenuation, it may be necessary to increase the power output of a transmitter to accommodate worst case conditions. This, however, results in excessive output levels during periods of reduced signal attenuation. Not only does use of a constant high power level waste power and result in increased maintenance, it may result in a receiver associated with a different transmitter/receiver pair experiencing interference particularly during those periods of reduced attenuation. The use of excess power unnecessarily enlarges the antenna beam contour of a transmitter and may result in unwanted signal distortion associated with such higher transmitter power levels. This distortion is because microwave systems previously mentioned generally employ high power amplifiers, as part of the signal transmission or transponder sections of the system. When the transmit power level is increased to overcome signal attenuation, the amplifier may be pushed into saturation where it will exhibit non-linear characteristics and distort the transmitted signal. This distortion is a primary impediment to reliable spectrally-efficient digital or other signaling using such amplifiers.
More particularly, when a signal containing amplitude variations is amplified, it will suffer distortion if the amplifier does not exhibit a linear amplitude transfer characteristic. This means that the output is not linearly proportional to the input. The signal will also suffer distortion if the phase shift introduced by the amplifier is not linear over the range of frequencies present in the signal, or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced further includes intermodulation of the components of the input signal. The products of the intermodulation appear (i) within the bandwidth of the signal causing additional undesirable distortion and (ii) extend outside the bandwidth originally occupied by the signal causing interference with adjacent channels and possibly violating licensing and regulatory spectral emission requirements. Although filtering can be used to remove some unwanted out-of-band distortion, this is not always practical, especially if the amplifier is required to operate on several different frequencies. Distortion products which are at multiples of the carrier frequency can also be produced in a nonlinear amplifier, although these can be removed by relatively simple filtering techniques.
Intermodulation is also a problem when multiple signals are amplified in the same amplifier even if, individually, the signals do not have amplitude variations. This is because the combination of the multiple signals produces amplitude variations as the various components combine with each other by adding and subtracting as their phase relationships change.
Amplifiers can introduce some distortion even if they are well designed. Perfect linearity over a wide range of amplitude is difficult to realize in practice. Moreover, as any amplifier nears its maximum output capacity, the output no longer increases as the input increases. At this point the amplifier is not linear. In fact, a typical amplifier becomes significantly nonlinear at a small fraction of its maximum power output capability. This means that, in order to maintain linearity, the amplifier is often operated at an input and output amplitude which is low enough such that the signals to be amplified are within the amplifier""s substantially linear transfer characteristic. This brute force approach reduces the drive level into the amplifier, so that the amplifier output power is considerably below saturation where the magnitudes of the distortion are tolerable. This method of operation is referred to as xe2x80x9cbacked offxe2x80x9d. While this technique has been found to be useful and has been widely employed with amplifiers, it loses a great deal of its appeal if the amplifier has to be backed off excessively in order to obtain acceptable distortion levels, since every dB of amplifier back off causes a loss in dB of radiated power. This method wastes power and requires that the amplifier be large and relatively expensive. Further operating in a xe2x80x9cbacked offxe2x80x9d mode is counter-productive to the previously mentioned method for boosting the power level to compensate for signal attenuation.
Another way to avoid distortion effects for digital modulation signaling is to use constant envelope type signals, such as unfiltered phase shift keying (PSK) or frequency shift keying (FSK) modulation. PSK and FSK signals are unaffected by non-linear distortion and the associated amplifiers can be smaller, run cooler, are more power efficient and less expensive. Unfortunately, such signaling schemes generally require a higher signal-to-noise ratio for a prescribed level of performance than other types of modulation (such as quadradture amplitude modulation (QAM)) that employ variations in amplitude to represent the data. Then too, many of the newer, bandwidth efficient modulation schemes use both amplitude and phase variations.
There is also a desire to be able to transmit multiple signals on different channels through a single amplifier. This reduces the number of separate amplifiers required and avoids the need for large, costly high level output signal combining filters which have undesirable power losses. This performance disparity between constant and non-constant amplitude signals increases in proportion to the data rate-signal bandwidth quotient (i.e., bits/sec/Hz). Accordingly, if the performance efficiency of a non-constant amplitude signal modulation scheme is to be obtained, compensation is necessary to account for amplifier distortion characteristics.
Another type amplifier used by microwave systems is the LINC (Linear Nonlinear Component) Amplifier. The LINC is based on generating amplitude variations in a signal by combining two signals which vary only in their relative phases. The vector sum of the two signals can represent any amplitude. Thus, it is possible to represent the instantaneous state of any signal or combination of signals. The phase and frequency of the component signals can also be made to represent that of the original so that when combined, the original signal is reconstructed. In spite of the fact that its theoretical efficiency can be very high, in a practical LINC transmitter the imbalance between the power gain and delay (or phase) of the two RF paths (especially for wideband applications) and the different non-linear characteristics of the two amplifiers limits the overall performance of the amplifier.
Because of these problems, various techniques have been developed for linearizing power amplifiers, correcting or compensating for nonlinearities using feedback, feed-forward, and predistortion processing.
Feedback is a mechanism in which a monitoring system looks at the output of the amplifier and attempts to alter the signal applied to the input of the amplifier so that it produces the intended output signal. This is arranged as a direct feedback loop. The delay in the feedback path means that the correction can be too late to offset unwanted distortions effectively, especially at higher bandwidths. Feedback reduces distortion at the expense of gain, often requiring further stages of amplification to produce the desired output.
Feedforward is widely used in commercial products which can amplify multiple signals and work over wide amplitude ranges. The method is quite complicated and the power efficiencies poor. Feedforward amplifiers are typically only 5% efficient. The complicated processing requirements also add to the cost and the power used and significant cooling capacity is required to remove waste heat. This is because the feedforward mechanism derives a signal which represents the inverse of the distortions produced by the amplifier by comparing the amplifier input and output to generate a difference signal representing a distortion signal. A small linear amplifier is used to amplify the distortion signal to the same level as that of the amplifier output. The amplified distortion signal is then subtracted from the main amplifier output. This method operates well over a wider bandwidth than, for example, the predistortion mechanism described below. However, balancing the amplitude and delay of the distortion signal so that it cancels the main amplifier errors exactly is complicated and costly to perform. For example, conventional feedforward arrangements require duplication of power amplifiers, one in each signal path, that are combined at the output. The amplifier duplication wastes power while the loss incurred by combining signals at the output reduces the effective output of the amplifier.
The predistortion type system attempts to compensate for the nonlinear transfer characteristic of an amplifier by forming an inverse model of its transfer characteristic. This transfer characteristic is applied to the low level signal at the input of the amplifier, to pre-distort the input signal such that, when it passes though the amplifier the signal emerges amplified and substantially undistorted. This method is capable of excellent results over a relatively small bandwidth. However, the predistortion mechanism has to be updated to account for variations in the amplifier transfer characteristic by monitoring the output and periodically updating the corrections. The filter also has to change its coefficients as often as every sample using the values stored in memory.
Therefore, there is a problem because existing amplifiers can be employed for linear amplification over only a relatively small range of input signal levels. Operation at or near saturation, while useful in attaining high values of efficiency, suffers from the foregoing forms of signal distortion.
Various implementations have been made to address the problems of the prior art using these techniques. For example, U.S. Pat. No. 4,291,277 to Davis describes a system which adapts itself to non-linearities of a power amplifier and predistorts the input signals. The system compensates in real or near real time (i.e., during the actual transmission of data) for distortion introduced by an amplifier during amplification. The digital signal, which is to be transmitted over microwave frequencies, is sent to a predistortion RAM and to a temporary storage buffer ROM. The predistortion RAM provides an input signal that will cancel the distortion characteristics of the amplifier. A portion of the amplifier output is fed back to a special receiver located with the transmitter which demodulates the signal and compares it to the original signal information sent to the ROM. The results of this comparison are used to update the predistortion RAM. The distortion-introducing behavior of the amplifier is continuously tracked. However, the method requires the use of a dedicated receiver circuit to demodulate and prioritize a sample of the amplifier signal.
Another implementation is described in a paper, entitled xe2x80x9cA New Technique for Adaptation of Linearizing Predistortersxe2x80x9d, presented at the 41st IEEE Vehicular Technology Conference in May 1991. In this implementation, Stapleton and Cavers use a pre-distorter which adjusts to the drifting characteristics in the power amplifier. This technique, however, does not provide real-time adaptation during transmission. Instead, the authors describe sampling based on out-of-band power, and which uses this scalar quantity to adjust the pre-distorter coefficients.
In the system described by Stapleton and Cavers a first receiver is set to constantly receive signals of frequency F2. A second receiver is used to make periodic checks of the out-of-band power by sending the received signals to a pre-distorter. Adjustments are then made to the pre-distortion coefficients. During normal operation, both receivers are set to F2. The verification of the transmitter is made on an intermittent basis so as not to interfere with the system operation. This system, however, does not transmit the distortion information using the communication channel and the method requires a dedicated receiver in the feedback loop.
Another example of a transceiver for the correction of amplified radio signals is described by Cyze et al., U.S. Pat. No. 5,740,520. FIG. 1 of the Cyze et al disclosure depicts a transceiver including a transmitter and first and second receivers. The transmitter and first receiver are conventional components of a transceiver, operating on different frequencies F1 and F2 respectively, for full duplex communication. The second receiver is used as a diversity channel signal at a frequency F3. Frequency F3 may be set to be the same as transmitter frequency F1 during the calibration process.
During normal communication, data is applied to a modem with the output of the modem supplied to a pre-distorter for correction prior to being transmitted. The output from the predistorter is supplied to the transmitter which comprises a quadrature modulator and a power amplifier. The modulator modulates the xe2x80x9cIxe2x80x9d (In phase) and xe2x80x9cQxe2x80x9d (Quadrature phase) components of baseband signals.
Cyze et al. further describes a transceiver which includes a transmitter for transmitting baseband signals at a first operating frequency. The transmitter also includes a lookup table (LUT) for storing correction coefficients for distorting the baseband signals, a pre-distorter for distorting the baseband signals and a receiver for receiving the baseband signals transmitted by a remote transmitter in a first mode and by the transmitter in a second mode. The receiver includes circuitry for processing the received baseband signals, a switching device for switching to the processing apparatus during the first mode and to the incoming signals to the lookup table up-dater during the second mode. The receiver also includes a lookup table up-dater, operative during the second mode, for receiving the signals transmitted by the transmitter and for updating the lookup table.
The lookup table, according to Cyze et al. stores both xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d coefficients for distorting the baseband signals and requires periodic updating to compensate for variations in the amplifier""s transfer characteristics. As in other prior art systems, a receiver must be dedicated to receiving update/calibration information (i.e., updates occur during standby, idle periods, or during no voice periods of normal communications).
The present invention provides a system and method which dynamically compensate for the non-linear behavior of power amplifiers used in digital data signal transmission systems over a wide range of gain and power output levels without the need for special purpose receiver circuitry, such as a receiver disposed at the transmitter. A preferred embodiment of the present invention utilizes signal measurements which may be made quickly, accurately, and without the need for expensive and complicated devices.
In one embodiment of the present invention, a predistortion system provides compensation for the non-linear behavior of an amplifier. The system may use a series of tables, such as for each fraction of dB throughout the operating range of an amplifier, that correlate the amplifier""s output power to correction coefficients for the xe2x80x9cIxe2x80x9d (In phase) and xe2x80x9cQxe2x80x9d (Quadrature phase) components of the baseband signal. The tables may include information regarding the amount of amplitude and phase compensation required to be applied to the xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d components of the baseband signal to compensate or xe2x80x9cnull outxe2x80x9d the distortion components introduced by the amplifier during or as a result of amplification. However, as the tables provide pre-distortion information related to the output power of the amplifier, such as by using the above mentioned series of tables indexed to output power level, a simple measurement of power at the output side of the amplifier can be used to adjust the predistorter.
In another embodiment of the present invention, a predistortion system provides for dynamic control of amplifier transfer characteristics, although the predistorter is placed in the circuit prior to signal attenuation or other amplified adjustment for power level control. Accordingly, the maximum output level of the predistortion circuitry is constant regardless of the actual signal power level desired for transmission. However, as the subsequent adjustment of signal amplified will affect the signal level in which the amplifier operates, the predistorter is adjustable in order to compensate for varying non-linearity in the operating range of the amplifier.
The predistortion system provides transmission such that the received distortion level experienced by the receiver of the transmitter/receiver pair is substantially reduced. Preferably, by detecting distortion in the signal at the receiver of the wireless link transmitter/receiver pair, the distortion introduced by the amplifier transfer characteristics may be compensated for. Moreover, the predistorter may compensate for the fluctuations in power level, circuit temperature and aging components, as dynamically experienced. Therefore, the distortion is lower under compensated-for conditions because the amount of predistortion is adjusted accordingly.
A closed feedback loop may be utilized to provide information regarding the distortion of a transmitted signal as experienced by the receiver of the transmitter/receiver pair. Accordingly, when the transmission signal is measured by the receiver, the system provides feedback to the transmitter in order to update tables of the preferred embodiment series of tables and to adjust the predistortion applied to the transmission signal. Such an embodiment allows for the series of tables to be updated to reflect nonlinear permanent changes to the output characteristic of the amplifier only as needed (i.e., infrequently), while utilizing the series of tables of the preferred embodiment to provide for predistortion adjustments typically requiring continuous feedback in prior art systems. This advantage of a preferred embodiment of the present invention is further synergized through the use of the preferred embodiment series of tables to compensate for environmental changes, thus eliminating the need for feedback regarding predistortion adjustment for such transient and inevitable changes in operating conditions.
When the receive signal is measured by the receiver to be at an acceptable level or to have acceptable characteristics, the system may provide appropriate feedback, or no response can be returned to the transmitter in order to maintain the current predistortion. For example, when the current predistortion table used is providing proper canceling of the amplifier-introduced distortion or when the transmitter""s amplifier is operating in the linear region of amplification these measurements and/or their associated feedback are not necessary. The receive distortion level characteristics may be measured at the receiver by determining the distortion of signal amplitude and phase.
Preferably the receiver system operates to make the desired measurements and initially format the measured attributes for use in predistortion adjustment. Accordingly, only a relatively small bandwidth return signal path is needed by the feedback loop, while still providing a desired level of functionality. For example, the receiver may not only measure signal characteristics, but also may conclude that an adjustment is necessary to compensate for this distortion and, in a very few reverse link control channel bits, communicate the information necessary for proper predistortion to the transmitter. Accordingly, wasting of reverse channel bandwidth may be avoided by not providing the adjustment circuitry of the transmitter with information regarding each measurement made of the particular parameter or parameters, while only returning the data calculated by the receiver to be necessary for proper adjustment of predistortion.
An aspect of the system is that the system uses the data transmission circuitry to transmit distortion information between the transmitter and the receiver, thus eliminating the need for a dedicated receiver.
An object of the present invention is that it reduces signal distortion.
An object of the present invention is that it eliminates the need for calibration circuitry.
The system and method adapts itself to non-linearities of a power amplifier in order to maintain desired performance levels (i.e., reduce the distortion level by forming an inverse model of its transfer characteristic). This transfer characteristic is applied to the low level signal at the input of the amplifier, to pre-distort it such that when it passes though the amplifier the signal emerges amplified and substantially undistorted. The compensation is accomplished without an additional or dedicated receiver circuit.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.